Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a fuel vapor canister containing an adsorbent, such as activated carbon. The stored vapors may then be purged from the canister during a subsequent engine operation. The purged vapors may be routed to engine intake for combustion, further improving vehicle fuel economy.
Fuel vapor adsorption by activated carbon is an exothermic reaction; the canister experiences an increase in temperature during canister loading. Conversely, fuel vapor desorption is endothermic, cooling the canister during purge events. Thus, a cool fuel vapor canister may have enhanced adsorption capacity, while a hot fuel vapor canister may have an increased ability to desorb fuel vapor. As such, heating the adsorbent is employed as a strategy to promote desorption and increase purge efficiency. Canister heating elements may directly heat the adsorbent, may heat the exterior of the canister, and/or may heat purge air passing through the canister. However, as part of the evaporative emissions system a canister heating element may be subject to periodic testing in order to meet emissions standards and regulations.
Other attempts to address canister heating element function include placing one or more thermocouples within the canister adsorbent bed. One example approach is shown by Hiltzik et al. in U.S. Patent Application 2008/0041226. Therein, selective regions of the canister are heated using electrical heating devices, while performance of the canister is monitored using thermocouples along the purge air flow path. However, the inventors herein have recognized potential issues with such systems. As an example, adding thermocouples incurs additional cost and complexity. Further, as part of the evaporative emissions system, any canister temperature sensors are also subject to additional diagnostic tests as part of emissions regulations.
In one example, the issues described above may be addressed by a method wherein a fuel vapor canister heating element is activated during a first condition, which includes an engine-off condition, and atmospheric air is directed through the fuel vapor canister and into an engine intake. Degradation of the fuel vapor canister heating element is indicated based on an output of an engine intake air temperature sensor. In this way, the integrity of the fuel vapor canister heating element can be determined without relying on canister temperature sensors, which may be confounded by the cooling of the fuel vapor canister during fuel vapor desorption.
As one example, a reversible vacuum pump coupled within an evaporative leak check module may be activated in a pressurizing mode to direct atmospheric air into the heated canister. Thus, the function of the canister heating element may be discerned using existing elements of the evaporative emissions system.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.